Field
This disclosure relates to rotatable polarizer devices for use in cylindrical waveguides.
Description of the Related Art
Satellite broadcasting and communications systems, such as Ku band very small aperture terminal (VSAT) communications systems, may use orthogonally polarized signals within the same frequency band for the uplink to and downlink from satellites.
A common form of antenna for transmitting and receiving signals from satellites consists of a parabolic dish reflector and a feed network where orthogonally polarized modes travel in a circular waveguide. Note that the term “circular” refers to the cross-sectional shape of the waveguide. An ortho-mode transducer may be used to launch or extract the orthogonal linearly polarized modes into or from the circular waveguides.
An ortho-mode transducer (OMT) is a three-port waveguide device having a common waveguide coupled to two branching waveguides. Within this description, the term “port” refers generally to an interface between devices or between a device and free space. A port may include an interfacial surface, an aperture in the interfacial surface to allow microwave radiation to enter or exit a device, and provisions to mount or attach an adjacent device.
The common waveguide of an OMT typically supports two orthogonal linearly polarized modes. Within this document, the terms “support” and “supporting” mean that a waveguide will allow propagation of a mode with little or no loss. In a feed system for a satellite antenna, the common waveguide may be a circular waveguide. The two orthogonal linearly polarized modes may be TE11 modes which have an electric field component orthogonal to the axis of the common waveguide. When the circular waveguide is partially filled with a dielectric material, the two orthogonal linearly polarized modes may be hybrid HE11 modes which have at least some electric field component along the propagation axis. Two precisely orthogonal TE11 or HE11 modes do not interact or cross-couple, and can therefore be used to communicate different information.
The common waveguide terminates at a common port aperture. The common port aperture is defined by the intersection of the common waveguide and an exterior surface of the OMT.
Each of the two branching waveguides of an OMT typically supports only a single linearly polarized TE10 mode. The mode supported by the first branching waveguide is orthogonal to the mode supported by the second branching waveguide. Within this document, the term “orthogonal” will be used to describe the polarization direction of modes, and “normal” will be used to describe geometrically perpendicular structures.
A satellite communications system may use a linearly polarized signal for the uplink to the satellite and an orthogonally polarized signal for the downlink from the satellite. The polarization directions for the uplink and downlink signals may be determined by the antenna and feed network on the satellite. To ensure maximum coupling of the signals to and from the satellite, each terrestrial antenna may include provisions to adjust the polarization directions of the uplink and downlink signals to exactly match the polarization directions defined at the satellite. In present antennas, the polarization directions of the uplink and downlink signals may be adjusted by rotating the entire antenna or by rotating all or portions of the feed network including the OMT. In either case, the item being rotated is heavy and the cables connecting to the feed network must be repositioned.
Elements in the drawings are assigned reference numbers which remain constant between the figures. An element not described in conjunction with a figure may be presumed to be the same as an element having the same reference number described in conjunction with a previous figure.
FIGS. 4A, 6, and 7 include dimensions defining a specific exemplary embodiment of a rotatable polarizing device. The specific embodiment is intended for use in the Ku frequency band from 13.75 GHz to 14.5 GHz, and was designed to satisfy a specific set of requirements. The specific embodiment is intended for use with a cylindrical waveguide having an inner diameter (WGID in FIG. 4A) of 0.526 inch. These dimensions are provided as a representative example of a rotatable polarizing device. Other embodiments of a rotatable polarizing device intended for use in other frequency bands and for other applications may have significantly different dimensions and are included in the disclosure herein.